New Data on Seawater Intrusion in Liepaja (Latvia) and Methodology for Establishing Background Levels and Threshold Values in Groundwater Body at Risk F5

Intensive water consumption in former decades caused formation of large depression cone near city Liepaja and resulted with seawater intrusion into Upper Devonian Mūru-Zagaru confined freshwater aquifer. Area affected by seawater intrusion is delineated as separate groundwater body at risk (F5) and according to Groundwater Directive threshold values for groundwater bodies at risk must be established to assess the status of a body and identify possible trends. Correct estimation of background levels is significant for determination of threshold values. This study shows an updated so called "BRIDGE" methodology for determination of background levels. A two-step approach how to establish background levels in much stricter manner is presented. Also, data on major ion chemistry, biogenic and trace elements in groundwater and seawater from sampling campaign in 2017 are displayed. Dataset include unique seawater sample taken from Baltic Sea. Finally, the calculated seawater fraction results in groundwater samples shows up to 50 % presence of seawater which decrease with increasing distance from the coast and increasing screen interval.